How do I find an original will

1) There is no way to find an earlier will other than by looking through  your MiL's papers, speaking to her close friends and family members, speaking to her lawyer or accountant (if she had such and you know who they are) and seeing if the will turns up or is in someone's possession. The problem is, wlls are not filed until a person passes away and someone seeks to probate it; there is no central respository or source for wills.
2) But even if there was an earlier will, an older will is superceded and replaced by a later one, if the later one is valid. So you would have to not just find an earlier will which you wish to be enforced, you'd need to invalidate the later will, too--see below.
3) IF you believe that your MiL was not mentally competent (e.g. due to Alzheimer's) when she created the new will, then you could challenge it and, if a court agrees, have the will thrown out; only mentally competent people can create valid and enforceable wills. If this happens, then a prior will (if one can be found) would be reinstated; or if there is no prior will, your MiL's estate would be distributed according to the rules for "intestate succession," or who gets what when there is no will. This would (to oversimply somewhat) mean dividing it evenly between her children.
But to show that your MiL was not competent when the will was made, you would have to have medical evidence (e.g. the testimony of doctors who examined or treated her at that time); without evidence, you could not prove your case.
Or you could try to show that the will was a forgery (not your mother's signature), but again would need evidence, such as from a handwriting analysis. 
You can sometimes show that the will was not valid because of "undue influence"--for example, that your MiL was at the time so totally dependent on her daughter that her daughter used her position of power and dominance to effectively and improperly bend the MiL to her will, and so the will was not the product of your MiL's own free will and voluntary choice. This requires evidence of your MiL's living situation and daughter's power over her at the time the will was made.
Or you may be able to show that will was not properly signed and witnessed and so failed to qualify as enforceable.
4) You write that the daughter passed before the MiL: depending on what the will says, it is not a given that whatever when to the daughter went to her own heirs or beneficiaries--some wills have the bequests to a "predeceasing" (dying earlier) beneficiary go to that beneficiary's owner beneficiaries and some don't. If the don't, they may go to other people named in the will, or they may go by intestate succession. So the will should be reviewed to see what happens in this case.
If you want to explore the option of challenging the will on one or more of the grounds above and what evidence you would need to do so, consult with a probate attorney.